In mammals, a large number of enzymes exist that metabolize drugs and other xenobiotics. Cytochrome P450s are among the most important of these enzymes and they are known to be involved in metabolism of most therapeutically-used agents. The P450s are also critical in the metabolic-activation of promutagens and procarcinogens. P450s known to metabolize xenobiotics are found in the CYP1, CYP2 and CYP3 families. Each of these families consist of two or more subfamilies containing the individual P450 forms. One question that has not been directly addressed is whether P450s are required for chemical carcinogenesis in an intact animal. The fact that P450s can metabolically-activate procarcinogens implies that they are involved in the process of chemical carcinogenesis. However, the only experiments suggestive of a role for P450s in cancer etiology are indirect chemically-induced transformation assays in cell culture, and genetic experiments in mice involving the Ah locus. However, no direct evidence is available to establish that P450s are necessary for carcinogenesis in an intact animal model system. To assess the physiological role, if any, for P450s and their potential contribution to the process of chemical carcinogenesis, P450-null mice were produced. CYP1A2 and CYP2E1-null mice were indistinguishable from mice having the P450s, indicating that they have no critical role in mammalian development or physiological homeostasis. Pharmacokinetic studies using caffeine showed rat CYP1A2 is responsible for metabolism of this drug and that metabolism is rate limiting for its urinary excretion. The CYP2E1-null mice were found to be resistant to the toxicity of acetaminophen and benzene as compared to normal mice indicating that CYP2E1 is the principal P450 involved in-metabolic activation of these chemicals. Carcinogenesis bioassays are currently in progress to determine the role of P450s in the hepatocarcinogenesis of various chemicals including 4-aminobiphenyl, food mutagens and phenacetin.